Extrusion surge suppressor and method

ABSTRACT

An apparatus and method for extruding an elastomeric polymer which is subject to periodic pressure surges. The apparatus includes a barrel having an upstream portion and a downstream portion, a hopper positioned for delivering polymer to the upstream portion of the barrel, a shaft rotationally mounted within the barrel and a drive means for rotating the shaft. The shaft has a conveying screw flight for conveying polymer downstream from the hopper through the barrel. The barrel has a discharge port located downstream of the hopper. A surge suppressor is provided on the shaft for reducing the pressure and flow rate surges in the polymer. The surge suppressor includes a screw flight downstream of the discharge port for urging the polymer upstream toward the discharge while permitting a portion of the polymer to flow downstream into the surge suppressor. A polymer seal is provided downstream of the surge suppressor.

BACKGROUND OF THE INVENTION

This invention provides an apparatus for reducing or eliminatingpressure and flow rate surges in polymer extrusion machines. It alsorelates to a method of extruding with a substantially constant polymerpressure. In particular, this invention provides an extrusion apparatusand method utilizing a surge suppressor incorporated into the extruderscrew.

FIELD OF THE INVENTION

Surges within polymer extruders are recognized as a major problem facedby the extrusion industry. Surges are output variations from an extruderscrew corresponding to variations in polymer pressure and changes inpolymer flow rate. Accordingly, surges are nearly synonymous in theextrusion industry with pressure and flow variations. Put simply, surgesare like waves wherein maximum output and pressure occur at the top ofthe wave and minimum output and pressure occur at the bottom of thewave. When a wave-like surge arrives at the discharge end of theextrusion screw, there will be a corresponding surge in dischargepressure and flow rate. Accordingly, an instantaneous pressure or flowrate surge will produce an instantaneous surge at the extrusion die.

Pressure and flow variations at the extrusion die are know to result indimensional variations in the extruded product. Such dimensionalvariations create severe problems, especially when it is desired ornecessary to extrude tube, rod or other shapes having tight tolerances.Dimensional variations may result in the extrusion of large quantitiesof expensive materials into useless products. Moreover, pressure andflow changes at the extrusion die cause dimensional variations along thelength of an extrusion. Inspection of one portion of the extrudedproduct may result in different results from other portions, reducingpredictability. Dimensional and other variations resulting from polymersurging ultimately results in material waste, product rejection, andother inefficiencies.

Various attempts have been made to efficiently and effectively controlvariations in extruder output. For example, valve control of extruderoutput was considered in Patterson et al, The Dynamic Behaviour ofExtruders, SPE ANTEC, pp. 483-487 (1978). Also, control of melttemperature and pressure by continuously varying screw speed coupledwith infrequent variations in die resistance was considered in Parnabyet al, Development of Computer Control Strategies for Plastic Extruders,Polym. Eng. Sci., Volume 15, No. 8, pp. 594-605 (1975).

Lee, in U.S. Pat. No. 4,118,163, also recognized difficulties incontrolling the uniformity of pumping zone pressure and attempted tominimize pressure and flow surges at the discharge end of an extruder.Lee provided a complicated screw extruder apparatus having two separatepumping zones, a first zone for feeding plastic from a hopper and asecond zone for pumping plastic back toward the first zone and out alateral exit orifice. The first and second zones were connected by acentral bore formed in the second zone of the screw which communicatedwith the first zone through radial passageways. The Lee extruder was anexpensive device requiring a highly specialized extruder screw.

These attempts failed to provide a practical and effective apparatus ormethod for reducing polymer surging. Accordingly, there is a great andthus far unsatisfied demand for a practical apparatus and method forreducing or eliminating surging of polymer in extrusion processes.

OBJECTS OF THE INVENTION

It is an object of this invention to provide an apparatus whichovercomes the problems associated with conventional extruders.

It is another object of the invention to provide an apparatus forreducing or eliminating surges of polymer during extrusion processes.

It is a further object of the invention to provide an extruder having anintegral surge suppressor.

It is another object of the invention to provide a surge suppressingextruder having an inexpensive and reliable means for reducing oreliminating surges before pressurized polymer reaches the extrusion die.

It is still another object of the invention to provide a method forreducing or eliminating surges known to occur in conventional extrusionprocesses.

It is a further object of the invention to provide a method for reducingor eliminating polymer surging by providing a modified extrusion screwwithin an otherwise standard extruder.

Other important objects of the invention will become apparent to one ofskill in this art in view of the following descriptions, the appendedfigures and the claims.

SUMMARY OF THE INVENTION

This invention provides an extruder having a barrel and a shaft mountedfor rotation within the barrel. The shaft has a conveying screw flightfor conveying and melting polymer pellets introduced into the barrelthrough a hopper. A metering screw flight meters the melted polymer anddelivers the polymer to an extrusion die. A surge suppressing screwflight on the shaft downstream of the die urges polymer toward the diewhile permitting a portion of the polymer to flow into or past the surgesuppressing screw flight. A seal is optionally provided near the surgesuppressing screw flight to prevent further polymer flow.

The surge suppressor absorbs instantaneous pressure and flow increases.The surge suppressor also compensates for instantaneous pressure andflow drops. Accordingly, the surge suppressor dampens pressure and flowsurges to maintain a substantially uniform pressure at the extrusiondie.

This invention also provides a method for reducing or eliminatingpolymer pressure and flow surges during extrusion processes. A shaft ofan extruder is provided with a surge suppressing screw flight. The surgesuppressing screw flight generates a pressure less than or equal to theextruder metering screw and a portion of the pressurized polymer flowsinto the surge suppressing screw flight. A seal is optionally provideddownstream of the surge suppressing screw flight to prevent furtherpolymer flowing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b are side views of conventional extrusion screws.

FIG. 2 is a side view of a segment of a conventional extrusion screwwithin a barrel.

FIGS. 3a and 3b are side views of two forms of extrusion screwsembodying features of this invention.

FIGS. 4a and 4b are side views of two more forms of extrusion screwsembodying features of this invention.

FIG. 5 is a side view of yet another form of extrusion screw embodyingfeatures of this invention.

FIG. 6 is a side view of still another form of extrusion screw embodyingfeatures of this invention.

DETAILED DESCRIPTION OF THE INVENTION

The following description is intended to refer to the specificembodiments of the invention illustrated in the drawings. Thisdescription is not intended to define or limit the scope of theinvention, which is defined separately in the claims that follow. Also,the drawings referred to throughout the following description are not toscale and are not intended to reflect actual dimensions or proportions.

FIGS. 1a and 1b are provided to illustrate features found inconventional extrusion screws utilized in conventional extruders. Theextrusion screws shown in FIGS. 1a and 1b both have an upstream portionto the right and a downstream portion to the left.

Referring to FIG. 1a, an extrusion screw S is driven from a drive end DRlocated at the upstream end of extrusion screw S. Just downstream ofdrive end DR, hopper pellets HP are introduced into the extruder barrel(not shown) within which extrusion screw S is rotationally mounted.Hopper pellets HP are conveyed downstream and melted into molten polymerin conveying and melting zone CM. Melted polymer is then metered inmetering zone M, sometimes referred to as a pumping zone, locateddownstream of conveying and melting zone CM. Melted polymer is thendischarged to an extrusion die through an axial discharge D.

Surges in pressurized melted polymer occur within metering zone M in theform of pressure surges and flow surges. Such surges are caused, asdescribed above, by extrusion screw rotation speed variations,variations in polymer temperature, variations in polymer supply, andother commonly encountered parameter changes. Such surges commonlyresult in pressure and flow rate surges at the extrusion die.

Referring to FIG. 1b, extrusion screw S is driven from a drive end DR atthe downstream end of extrusion screw S. Hopper pellets HP areintroduced into the barrel (not shown) and are conveyed and melted inconveying and melting zone CM at the upstream portion of extrusion screwS. Melted polymer is metered in metering zone M just upstream of aradial extrusion die discharge D.

In order to prevent pressurized polymer from flowing downstream ofextrusion die discharge D and into a transmission mechanism (not shown)at drive end DR, a dynamic seal DS is provided downstream of extrusiondie discharge D. Seals similar to dynamic seal DS, also known in theindustry as seal screws or viscous seals, are commonly used on gearpumps and on some drum extruders. Dynamic seals are also used ininternal mixers and vertical single screw extruders to keep polymer meltaway from critical parts of process machinery.

Most extruders drive the extrusion screw from the end opposite theextrusion die. In other words, the extrusion screw begins at thetransmission at an upstream portion and terminates in a point at theopposite, downstream end (see FIG. 1a). An example of such an extrusionscrew was illustrated by Adderley, Jr., in U.S. Pat. No. 4,465,451.

Other extruders drive the extrusion screw from its downstream portionand have an extrusion die discharge between the upstream and downstreamends of the screw. This type of extrusion screw S is shown in FIG. 1b.An example of such an extruder was also illustrated by Li et al., inU.S. Pat. No. 4,695,240. Dynamic seals have been used in such extrudersto prevent melted polymer from entering and fouling transmissionmechanisms attached to drive the screw.

Referring to FIG. 1b, dynamic seal DS prevents polymer flow pastextrusion die discharge D to drive end DR. Accordingly, dynamic seal DSis designed to maximize pressure so as to generate pressure greater thanthat developed in metering zone M. Because the pitch of the screw indynamic seal DS is opposite that of metering zone M and the screw isdesigned to generate maximum pressure, dynamic seal DS pumps meltedpolymer back upstream to the extrusion die and seals against downstreampolymer flow.

A variety of extrusion screws, many of which included dynamic or viscousseals, were disclosed in the following patents: Geier et al., U.S. Pat.No. 3,023,455; Kasting et al., U.S. Pat. No. 3,632,256; Latinen, U.S.Pat. No. 3,797,550; Okada et al., U.S. Pat. No. 3,802,670; Shinmoto,U.S. Pat. No. 3,924,841; Markel et al., U.S. Pat. No. 4,689,187;Kolossow, U.S. Pat. No. 4,730,935; Shogenji et al., U.S. Pat. No.4,766,676; Pena, U.S. Pat. No. 4,966,539; and Klein, U.S. Pat. No.5,106,286. The extrusion screw shown in U.S. Pat. No. 3,924,841,incorporated herein by reference, has a reverse thread portion whichserves to force back the molten resin toward the mixing zone to preventpolymer leakage past the extruder screw shank.

FIG. 2 illustrates structural elements of conventional extrusion screws.Extrusion screw S has a flight F helically arranged at a helix angle H.Flight F is also known as a thread or screw. Flight F has a flight widthFW. The space between adjacent flights defines a channel C betweenextrusion screw S and an extruder barrel B. Channel C has a channeldepth CD (sometimes known as thread depth) and a channel width CW.Extrusion screw S has a shaft diameter D_(s) and extrusion screw S issized to fit within barrel B having a barrel diameter D_(b).

Rotation of extrusion screw S shown in FIG. 2 conveys polymer (notshown) in channel C in a downstream direction. For example, rotatingextrusion screw S in a counter-clockwise direction from the right-handside of FIG. 2 conveys polymer toward the left-hand side of FIG. 2.

Extrusion screws can of course be provided with a wide variety ofdimensions, configurations and shapes. Meyer, in U.S. Pat. No.5,215,374, illustrated a variety of extrusion screw shapes.

FIGS. 3a, 3b, 4a, 4b, 5 and 6 illustrate several embodiments of theextrusion surge suppressor according to this invention. This inventionis not, however, limited to the embodiments illustrated in the figures,but instead is defined separately in the appended claims.

Referring to FIG. 3a, an extrusion screw is provided with a meteringzone M downstream from a conveying and melting zone (not shown) intowhich hopper pellets are introduced. Downstream of metering zone M is aradially extending extrusion die discharge D. Farther downstream fromextrusion die discharge D is a surge suppressor SS for suppressingmelted polymer pressure and flow rate surges. Surge suppressor SS inthis embodiment generates a polymer pressure less than that generated inmetering zone M, thereby allowing some melted polymer to flow downstreamthrough surge suppressor SS. Surge suppressor SS is in the form of ascrew flight having a direction opposite that in metering zone M.Accordingly, surge suppressor SS pumps a substantial portion of meltedpolymer back toward metering zone M and extrusion die discharge D.Details of a preferred surge suppressor SS are provided below.

A dynamic seal DS is provided on extrusion screw S downstream from surgesuppressor SS. Dynamic seal DS seals against downstream flow of themelted polymer that passes through surge suppressor SS. Dynamic seal DSis formed from a helical groove cut into extrusion screw S in adirection opposite to the screw flights in metering zone M. Dynamic sealDS generates a high polymer pressure greater than that generated inmetering zone M.

To generate high polymer pressure, dynamic seal DS is provided with asmall helix angle H (FIG. 2), a shallow channel depth CD and/or a narrowchannel width CW. Dynamic seal DS is preferably formed with a smallaxial length to permit a shorter extrusion screw S. Dynamic seal DS isprovided with a helix angle H not exceeding about half that of meteringzone M. Channel depth CD in dynamic seal DS does not exceed about halfthat of metering zone M. Also, the axial length of dynamic seal DS isless than or equal to about 25% that of metering zone M. Finally,channel width CW in dynamic seal DS does not exceed about 10% of thescrew diameter. A dynamic seal DS so designed generates a pressure muchgreater than metering zone M and prevents flow of melted polymer to thedrive end DR of extrusion screw S and into the screw drive mechanism(not shown) attached to drive end DR.

Referring to FIG. 3b, an extrusion screw S is similar to that shown inFIG. 3a except that extrusion screw S is driven from a drive end DR atthe upstream end of extrusion screw S. Drive end DR is provided upstreamfrom a conveying and melting zone CM into which hopper pellets (notshown) are introduced. Downstream from conveying and melting zone CM isa metering zone M for metering melted polymer and delivering the polymerto a radially extending extrusion die discharge D. Downstream fromextrusion die discharge D is a surge suppressor SS similar to thatdescribed with reference to FIG. 3a. As in FIG. 3a, the surgesuppressing screw embodiment shown in FIG. 3b has a dynamic seal DSlocated downstream from surge suppressor SS. Dynamic seal DS has astructure similar to that described with reference to FIG. 3a. Dynamicseal DS prevents downstream flow of melted polymer that flows throughsurge suppressor SS. Accordingly, dynamic seal DS prevents flow ofpressurized melted polymer downstream into downstream portions of barrelB.

FIGS. 3a and 3b both illustrate embodiments having a surge suppressorpermitting downstream flow of some polymer with a dynamic seal whichseals against farther downstream flow. FIG. 3a shows such an embodimentdriven from a downstream end of the extrusion screw. FIG. 3b shows anembodiment wherein the extrusion screw is driven from its upstream end.

Referring to FIG. 4a, another extrusion screw embodiment is providedwith a metering zone M downstream from a conveying and melting zone (notshown) into which hopper pellets are introduced. Metering zone M metersmelted polymer and delivers it to a radially extending extrusion diedischarge D. Downstream from extrusion die discharge D is a surgesuppressor SS similar to those shown in FIGS. 3a and 3b. Surgesuppressor SS pumps a portion of the melted polymer back upstream towardmetering zone M and extrusion die discharge D. Surge suppressor SS alsopermits downstream flow of a portion of melted polymer.

The portion of melted polymer that flows downstream past surgesuppressor SS exits barrel B through a radially extending polymerdischarge port 10. Radial discharge port 10 may also take the form of ableed hole for the escape of small amounts of melted polymer. Radialpolymer discharge port 10 optionally leads to a restriction valve (notshown) or any other known means of restricting melted polymer flow.Discharge of the portion of polymer that passes downstream through surgesuppressor SS and out port 10 prevents fouling of transmissionmechanisms at a drive end DR at the downstream end of extrusion screw S.

Referring to FIG. 4b, an extrusion screw S similar to that shown in FIG.4a is shown, differing mainly in that extrusion screw S in FIG. 4b isdriven from a drive end DR at the upstream end of extrusion screw S.Downstream from drive end DR is a conveying and melting zone CM intowhich hopper pellets are introduced. Melted polymer is metered in ametering zone M downstream from conveying and melting zone CM fordelivery to a radially extending extrusion die discharge D. Downstreamfrom extrusion die discharge D is a surge suppressor SS similar to thatshown in FIG. 4a.

The portion of melted polymer that flows downstream through and pastsurge suppressor SS exits barrel B through an axial polymer discharge20. Axial polymer discharge port 20 optionally terminates at arestriction valve or other known restriction device. Port 20 may also bereferred to as a bleed hole. Accordingly, the small portion of meltedpolymer that flows downstream from surge suppressor SS exits theextruder through port 20 while the majority of melted polymer exitsextrusion die discharge D upstream from surge suppressor SS.

FIGS. 4a and 4b both show extrusion screw embodiments wherein a surgesuppressor which permits downstream passage of melted polymer isprovided in conjunction with a bleed hole and optional restrictiondevice. FIG. 4a shows an embodiment having a surge suppressor combinedwith a radially extending bleed hole. FIG. 4b shows an embodiment havinga surge suppressor combined with an axially extending bleed hole.

Referring to FIG. 5, yet another embodiment of an extrusion surgesuppressor according to this invention is illustrated. This embodimentprovides an extrusion screw S having a metering zone M downstream from aconveying and melting zone (not shown) which supplies melted polymer.Metering zone M meters and delivers pressurized polymer to a radiallyextending extrusion die discharge D. Downstream from extrusion diedischarge D is a surge suppressor SS. Surge suppressor SS pumps somemelted polymer back upstream toward metering zone M and extrusion diedischarge D while permitting a portion of melted polymer to flow fartherdownstream through surge suppressor SS.

An O-ring 30 is captured within an O-ring groove 40 formed in barrel B.O-ring 30 prevents downstream flow of the melted polymer that passesthrough surge suppressor SS. O-ring 30 is preferably formed from anyknown elastomeric material. Whatever material is selected, however,O-ring 30 should be capable of withstanding the elevated temperaturesmaintained during extrusion processes.

O-ring 30 provides a circumferential seal against an outermost surfaceof O-ring groove 40 and a circumferential seal against the surface ofextrusion screw S. These seals provided by O-ring 30 prevent passage ofmelted polymer to the drive end DR of extrusion screw S, therebypreventing fouling of any transmission mechanism attached to drive endDR.

The extrusion surge suppressor embodiment shown in FIG. 5 has a surgesuppressor in combination with an O-ring seal which seals-off thepolymer that flows through the surge suppressor. It is of coursecontemplated (although not shown) that the screw in FIG. 5 could also bedriven from a drive end located at the upstream end of the screw. It isalso contemplated that any other known mechanical seal device can besubstituted for O-ring 30 and O-ring groove 40.

Referring to FIG. 6, an extrusion screw S is again provided with ametering zone M which receives melted polymer from a conveying andmelting zone (not shown). Metering zone M meters and deliverspressurized and melted polymer to radially extending extrusion diedischarge D. Downstream from extrusion die discharge D is provided asurge suppressor SS similar to that shown in FIG. 5. Surge suppressor SSpumps a portion of melted polymer back towards metering zone M and outextrusion die discharge D. Another portion of the melted polymer flowsdownstream through at least a portion of surge suppressor SS.

A coolant reservoir 50 is provided within barrel B at a position whichpreferably overlaps with surge suppressor SS on extrusion screw S.Coolant is circulated through coolant reservoir 50 to cool the meltedpolymer in a portion of surge suppressor SS. As the melted polymer iscooled, it tends to prevent farther downstream flow. Accordingly, theportion of melted polymer which flows downstream through a portion ofsurge suppressor SS is sealed against flowing farther downstream,thereby preventing fouling of screw transmission mechanisms attached atdrive end DR.

The embodiment shown in FIG. 6 illustrates the combination of a surgesuppressor with polymer cooling to reduce or eliminate polymer surgingwhile preventing polymer leakage. It is of course contemplated thatdrive end DR could also be located at the upstream end of extrusionscrew S. It is also contemplated that coolant reservoir can besubstituted for any known cooling means, including but not limited to acoiled coolant flow passage or even convection cooling induced by airflow around or through the extruder barrel. Also, coolant reservoir 50or any other known cooling means can be positioned to coincide withsurge suppressor SS, can overlap with surge suppressor SS or can bepositioned downstream of surge suppressor SS.

Operation of the extrusion surge suppressor according to this inventionwill now be described with reference to FIGS. 2 and 3a. In essence, thesurge suppressor portion of the extrusion screw provides a uniformoutput pressure at the extrusion die by absorbing surges in polymerpressure and flow rate. In conventional extruders, surges are known tooccur when the extruder speed is increased or when other extrusionsparameters such as temperature are varied. Such changes result influctuations in output. Accordingly, in conventional extruders, anychange in extruder output is transmitted directly to the extruder die,thereby causing the severe disadvantages described above.

The extrusion surge suppressor of this invention eliminates the peaksand valleys of wave-like surges to provide an output that is uniform.More specifically, the surge suppressor acts to store surging polymerassociated with pressure or flow rate increases so that excess polymerdoes not travel directly from the metering zone to the extrusion die.Accordingly, the surge suppressor absorbs the surge while preventingtransmission of the surge directly to the extrusion die. When, on theother hand, the surge represents a pressure drop or flow reduction, thesurge suppressor gives up some of its stored polymer to the extrusiondie discharge to even-out the die output.

The surge suppressor is preferably formed with a length sufficient toallow molten polymer pressure generation approaching that of themetering zone. The surge suppressor's ability to generate pressureincreases with length. As polymer enters the surge suppressor (pushedinto the surge suppressor by pressure generated in the metering zone),the pressure in the surge suppressor approaches the pressure in themetering zone. Accordingly, a surge of molten polymer flows into thesurge suppressor before it reaches the discharge die.

It is believed that the function of the surge suppressor and methodaccording to this invention is founded upon fundamentals of polymerflow. In a steady state the drag flow of polymer in the surge suppressorrelates to the pressure flow of the polymer according to the followingequations: ##EQU1## wherein quantity (1) is polymer drag flow in thesurge suppressor and quantity (2) is polymer pressure flow. CW and CDare defined in FIG. 2. P is the pressure developed in the screw meteringzone, z is the helical length over which pressure P is developed in thesurge suppressor, and v_(sb) is the relative velocity between theextruder screw and extruder barrel. In steady state operation of thesurge suppressor, drag flow and pressure flow are approximately equal:##EQU2##

According to relationship (3), there is an increase in flow into thesurge suppressor whenever an instantaneous pressure increase occurs atthe surge suppressor entrance. This flow increase causes an increase infilled length z. If the pressure increase is maintained for sufficienttime, a new equilibrium will be reached with the new filled length zuntil drag flow is again proportional to pressure flow.

If molten polymer is presumed to be incompressible, an instantaneouspressure surge will be accompanied by an instantaneous flow rateincrease. Such an instantaneous flow rate surge is absorbed by the surgesuppressor of this invention. The initial flow surge into the surgesuppressor is large and then gradually tapers. Accordingly, initial flowincrease into the surge suppressor immediately reduces flow into thedie, thereby reducing output variations at the extrusion die. In otherwords, a step change in pressure or flow rate in the metering zone willnot produce a step change in the discharge pressure or flow rate at theextrusion die when a surge suppressor according to this invention isused.

If the duration of the surge is very short (less than five seconds, forexample), flow into the surge suppressor and pressure in the extrusiondie will still be building before the surge ends. Accordingly, pressureand flow will start to reduce even before a new steady state is reachedand the amplitude of the pressure and flow surge is dramatically reducedor eliminated.

It has been discovered that it is easiest to suppress surges when thetime required for flow into the surge suppressor to reach equilibrium(Δt_(eq)) is longer than the time duration of the surge (Δt_(surge)).For example, surges having a duration as long as an hour can be limitedby pressure feedback control. Accordingly, it is most preferable todesign the surge suppressor according to this invention for reduction ofshort-term surges lasting only a few seconds.

Preferred Embodiment

Surge suppressors according to this invention are preferably designed tomaximize polymer volumetric capacity so as to absorb larger pressure andvolumetric surges. This is preferably accomplished by adjusting theflights in the surge suppressor by optimizing channel width CW, channeldepth CD, and helix angle H (FIG. 2).

Surge suppressors according to this invention generate less pressurethan dynamic seals, such as the one described above with reference toFIG. 3a, which are intended to seal against polymer flow. As compared todynamic seals, the surge suppressor of this invention will have agreater helix angle H (FIG. 2), a greater channel depth CD and/or awider channel width CW.

Wider channels provide increased polymer storage capacity during asurge. Increased polymer storage capacity also allows the surgesuppressor to pump back into the extrusion die a greater volume ofpolymer during a pressure or flow rate drop. Another advantage of widersurge suppressor channels is that the overall axial length of the surgesuppressor can be made smaller (because fewer channels are required) andallows the manufacture of a small extruder. Accordingly, the channelwidth of the preferred surge suppressor embodiment is greater than about10% of the screw diameter. Referring to FIG. 2, channel width CW in thesurge suppressor is preferably greater than about 10% of barrel diameterD_(b).

The most preferable surge suppressor according to his inventiongenerates lower pressures than that generated in the metering zone.Lower pressure generation maintains continuous flow of molten polymerdownstream through the surge suppressor. Such continuous flowreplenishes molten polymer in the surge suppressor, thereby preventingpolymer degradation and burning. As described above, the flow of moltenpolymer downstream from the surge suppressor can be stopped with anysealing method or simply allowed to flow from the extruder barrel. Toachieve these benefits, the preferred surge suppressor embodimentgenerates a pressure less than the metering zone and, most preferably,will only be capable of generating a pressure up to approximately 95% ofthe pressure generated in the metering zone.

A surge suppressor according to this invention is preferably formed witha helix angle H (FIG. 2) larger than that of the dynamic seal in FIG.3a, or greater than about half that of the metering zone screw flight.Helix angle H of the surge suppressor is most preferably slightly largerthan the helix angle H in the metering zone. Where the other surgesuppressor dimensions coincide with those in the metering zone, thepreferred surge suppressor embodiment has a helix angle H about 10%larger than the metering zone. Such a helix angle has been discovered togenerate a maximum pressure of about 90% the pressure generated in themetering section if the other dimensions are the same.

A surge suppressor according to this invention preferably has a channeldepth CD larger than that of the dynamic seal in FIG. 3a, or greaterthan about half that of the metering zone. If helix angle H were thesame as in metering zone M, the most preferred surge suppressor wouldhave a channel depth CD about twice that in the metering zone. Such achannel depth CD has been discovered to generate about 25% the pressuregenerated in the metering zone.

The axial length of the surge suppressor according to this invention ispreferably greater than that of the dynamic seal of FIG. 3a, or greaterthan about 25% that of the metering zone. However, axial length of thesurge suppressor is also preferably less than that of the metering zone.If channel depth CD and helix angle H were both the same in the surgesuppressor as in the metering zone, reducing the surge suppressor lengthto about 95% of the metering zone length would generate about 95% of themetering zone pressure.

Of course, any combination of channel depth CD, channel width CW, surgesuppressor length and helix angle H can be used so long as the surgesuppressor retains its surge suppressing function. However, it is mostpreferable that the surge suppressor is not designed to generate morethan about 95% of the metering zone pressure.

In any embodiment, the surge suppressor according to this inventionprovides significant benefits. The surge suppressor dramatically reducesor eliminates pressure and volumetric surges commonly known to occur inconventional extruders. The surge suppressor prevents these surges fromtransferring directly to the extrusion die, thereby reducing oreliminating variations in extruder product dimensions and quality. Also,the surge suppressor according to this invention is practical andinexpensive, merely requiring modification of the extrusion screwwithout complicated and expensive control systems such as thoseconsidered in the past.

These surprising and significant benefits are conferred withoutdisadvantage. The continuous flow of molten polymer through the surgesuppressor prevents degradation or burning of the polymer. Also, thesurge suppressor can be provided without requiring a significantincrease in extruder length. Accordingly, the surge suppressor andmethod according to this invention provides a simple, effective andpractical solution to the longstanding problem of surging in extruders.

Many modifications to the surge suppressor embodiments described hereincan be made without departing from the spirit and scope of thisinvention. For example, instead of forming helical grooves in theextrusion screw to produce the surge suppressor, a surge suppressingresult can be produced by creating helical grooves in the cylindricalhousing or barrel. Also, the surge suppressor is optionally a separatelydriven component. It is the relative motion between the shaft and thebarrel that creates the important pumping effect of the surgesuppressor.

Also, more than one bleed hole can be provided for the embodiments shownin FIGS. 4a and 4b and any known conventional seal or restriction devicecan be used in conjunction with the surge suppressor in any embodiment.Of course, multiple seals can be used in combination if desirable ornecessary. For example, a dynamic seal can be used in conjunction withpolymer cooling.

It is also contemplated that bleed holes 10 and 20 (FIGS. 4a and 4b,respectively) may also communicate with a flexible membrane covering thehole or a piston inserted into the hole as opposed to other forms ofrestriction device. Such a flexible membrane or piston could providesome additional surge reduction and/or sealing capability.

The screw dimensions and configuration can be varied in any way in anycombination in the surge suppressor so long as the surge suppressingfunction is maintained. It is also contemplated that the channel widthCW, channel depth CD and helix angle H may vary over the surgesuppressor's length. Although the shaft diameter Ds and is preferablyconstant throughout the surge suppressor, shaft diameter Ds (FIG. 2) mayoptionally be tapered.

Many other modifications will be apparent to those of skill in theextrusion art. Such modifications are within the scope of thisinvention, which is defined in the following claims.

What is claimed is:
 1. An apparatus for extruding a polymer which issubject to periodic surges of pressure or flow rate, said apparatuscomprising:a barrel having an upstream portion and a downstream portion;a hopper positioned and connected for delivering said polymer to saidupstream portion of said barrel; a shaft mounted for rotational movementabout a longitudinal axis within said barrel; drive means connected forrotating said shaft; said shaft having a conveying screw flight and ametering screw flight arranged for conveying and metering said polymerdownstream from said hopper through said barrel, wherein said surges aredeveloped within said barrel; said barrel having a discharge portlocated downstream of said hopper; a surge suppressor connected to saidshaft for reducing said surges in said polymer, said surge suppressorincluding a reverse screw flight on said shaft and located downstream ofsaid discharge port for urging said polymer upstream toward saiddischarge port while permitting a portion of said polymer to flow in adownstream direction into said surge suppressor, said surge suppressorbeing capable of generating a pressure less than or substantially equalto a maximum pressure generated in said metering screw flight; andsealing means located downstream of said surge suppressor.
 2. Theapparatus for extruding a polymer described in claim 1, wherein saiddrive means is connected to a downstream portion of said shaft at aposition downstream from said sealing means.
 3. The apparatus forextruding a polymer described in claim 1, wherein said drive means isconnected to an upstream portion of said shaft at a position upstreamfrom said hopper.
 4. The apparatus for extruding a polymer described inclaim 1, wherein said sealing means is a dynamic seal.
 5. The apparatusfor extruding a polymer described in claim 1, wherein said sealing meansincludes a bleed hole in said barrel.
 6. The apparatus for extruding apolymer described in claim 1, wherein said sealing means is a mechanicalseal device contacting an outside surface of said shaft.
 7. Theapparatus for extruding a polymer described in claim 6, wherein saidmechanical seal device is an O-ring.
 8. The apparatus for extruding apolymer described in claim 1, wherein said sealing means is a polymercoolant means.
 9. The apparatus for extruding a polymer described inclaim 8, wherein said polymer coolant means is positioned to cool saidpolymer in at least a portion of said surge suppressor.
 10. Theapparatus described in claim 1, wherein said reverse screw flight insaid surge suppressor has a helix angle greater than about half that ofsaid metering screw flight.
 11. The apparatus described in claim 1,wherein said reverse screw flight in said surge suppressor has an axiallength greater than about 25% that of said metering screw flight. 12.The apparatus described in claim 1, wherein said reverse screw flight insaid surge suppressor has a channel depth greater than about half thatof said metering screw flight.
 13. The apparatus described in claim 1,wherein said reverse screw flight in said surge suppressor has a channelwidth greater than about 10% of an inside diameter of said barrel.
 14. Amethod for extruding a polymer using an extruder having a barrel with adischarge, a hopper communicating with an upstream portion of saidbarrel for delivering said polymer, a shaft rotationally mounted withinsaid barrel and a drive means for rotating said shaft, said methodcomprising the steps of:supplying polymer pellets through said hopperand into said barrel; melting said polymer pellets to produce meltedpolymer; conveying said melted polymer through said barrel with aconveying screw flight; metering said melted polymer in said barrel witha metering screw flight, thereby generating a metering pressure and ametering flow rate; suppressing surges in said metering pressure or saidmetering flow rate by generating a suppressing pressure less than orequal to a maximum metering pressure with a reverse screw flightpositioned downstream of said discharge; sealing against flow of saidmelted polymer downstream from said reverse screw flight; and directingsaid melted polymer from said barrel between said metering screw flightand said reverse screw flight to said discharge.
 15. The method of claim14, wherein said step of sealing against flow of said melted polymerincludes providing a dynamic seal downstream from said reverse screwflight.
 16. The method of claim 14, wherein said step of sealing againstflow of said melted polymer includes providing a bleed hole downstreamfrom at least a portion of said reverse screw flight.
 17. The method ofclaim 14, wherein said step of sealing against flow of said meltedpolymer includes providing a mechanical seal between an outside surfaceof said shaft and an inner surface of said barrel.
 18. The method ofclaim 14, wherein said step of sealing against flow of said meltedpolymer includes providing means for cooling said melted polymerdownstream from at least a portion of said reverse screw flight.
 19. Themethod of claim 14, wherein said step of suppressing surges includesproviding said reverse screw flight with a helix angle greater thanabout half that of said metering screw flight.
 20. The method of claim14, wherein said step of suppressing surges includes providing saidreverse screw flight with an axial length greater than about 25% that ofsaid metering screw flight.
 21. The method of claim 14, wherein saidstep of suppressing surges includes providing said reverse screw flightwith a channel depth greater than about half that of said metering screwflight.
 22. The method of claim 14, wherein said step of suppressingsurges includes generating a suppressing pressure not exceeding about95% of said metering pressure.
 23. The method of claim 14, wherein saidstep of suppressing surges includes providing said reverse screw flightwith a channel width greater than about 10% of an inside diameter ofsaid barrel.
 24. A method for suppressing pressure or flow rate surgesduring extrusion of a polymer using an extruder having a barrel, a shaftrotationally mounted within said barrel and a drive means for rotatingsaid shaft, said shaft having a conveying and melting zone, a meteringzone and a surge suppressing zone, said method comprising the stepsof:supplying said polymer through a hopper into an upstream portion ofsaid barrel; conveying and melting said polymer in said conveying andmelting zone; metering said polymer in said metering zone using a screwflight, thereby generating a polymer pressure and a polymer flow rate;providing a discharge port located downstream of said metering zone andupstream of said surge suppressing zone; urging said polymer upstreamtoward said discharge port through said surge suppressing zone using areverse screw flight while permitting a portion of said polymer to flowdownstream through said surge suppressing zone, thereby reducing saidpressure or flow rate surges in said polymer pressure, by generating apressure in said suppressing zone less than or substantially equal to amaximum pressure generated in said metering zone; and sealing saidpolymer against flow downstream from said surge suppressing zone.